1. Field of the Invention
The present invention relates to an antenna device preferably for use in an RFID system or short-range wireless communication system that communicates with partner equipment through an electromagnetic field signal.
2. Description of the Related Art
An antenna device for use in an HF-band communication system, such as Felica (registered trademark) or NFC, is disclosed in Japanese Unexamined Patent Application Publication No. 2002-325013. FIG. 1 is a front view that illustrates the structure of the antenna device described in Japanese Unexamined Patent Application Publication No. 2002-325013.
An antenna coil 30 illustrated in FIG. 1 includes an air-core coil 32 including spirally wound conductors 31 (31a, 31b, 31e, 31d) in a plane on a film 32a and a flat-shaped magnetic core member 33 disposed in the air-core coil 32 so as to be positioned substantially in parallel with the plane of the air-core coil 32. The air-core coil 32 has an opening 32d in which the magnetic core member 33 is disposed. The first terminal 31a and the coupling conductor 31e are coupled to each other with a through hole 32b. The second terminal 31b and the coupling conductor 31e are coupled to each other with a through hole 32c. This magnetic antenna is arranged on a conductive plate 34.
The antenna device in which the magnetic body is disposed in the opening of the antenna coil illustrated in FIG. 1 can control the directivity of the antenna by controlling the distribution of magnetic fields occurring in the antenna coil. Although depending on the surrounding environment (e.g., relationship with the ground and the like), the communication distance mainly toward the axial direction of the magnetic body can be extended.
In such an antenna device, however, because it has a structure in which the antenna coil has an opening in its central portion and a bar-shaped magnetic body (flat-shaped magnetic core member) is disposed in this opening, its manufacturing process is complicated. Thus, the positional accuracy between the magnetic body and the antenna coil is low and the characteristics of an obtained antenna device tend to widely vary. In addition, an adhesive for use in fixing the antenna coil and the plate-shaped magnetic body is needed, and the adhesive may negatively affect the electric characteristics of the antenna device, depending on the type or applied amount of the adhesive.
In addition, because each of the conductors 31 (31a, 31b, 31e, 31d) is disposed in the plane on the film 32a, a gap is present between the conductor 31 and the magnetic core member 33. Thus, there is a problem in that a magnetic flux is reflected on the interface between the magnetic body and a dielectric body (non-magnetic body) and the magnetic-field coupling decreases.